In an exhaust gas recirculation system, exhaust air is supplied to a surge tank or an intake branch pipe, thereby mixing the exhaust air with intake air (For example, refer to P.3 and FIGS. 2 and 3 of JP H7-259656A, P.3 and FIG. 5 of JP H7-247917A, and P.5 and FIG. 11 of JP H11-210560A). According to JP H7-259656A, exhaust air is introduced into intake air from circular and oval openings which open to a surge tank, thereby uniformly distributing the exhaust air to each cylinder.
According to JP H7-247917A, an exhaust gas recirculating path, guiding the exhaust air to an upstream of a swirl control valve, is formed to be an arcwise oval, thereby increasing the temperature in an intake port to facilitate fuel vaporization. According to JP H11-210560A, an oval shaped exhaust air intake port, which is elongated in a flow direction, is formed immediately below a throttle valve, thereby mixing intake air with exhaust air uniformly and preventing deposit adherence.
Water condensation may occur during exhaust air circulation due to cooling of vapor contained in the exhaust air. If the condensed water accumulates in an exhaust introducing path leading to intake air, the exhaust introducing path is blocked off. Consequently, the introduced exhaust airflow is not uniformly mixed with the intake air and tends to be intermittent. These phenomena increase difficulties in uniform diffusion of the exhaust air into the intake air and uniform distribution to each cylinder. As a result, the exhaust gas recirculation is not smoothly conducted. For this reason, it is necessary to discharge the condensed water smoothly from the exhaust introducing path by flowing the condensed water along a wall surface of the exhaust introducing path.
In JP H7-259656A, JP H7-247917A, and JP H11-210560A, condensed water, occurring by a time that the exhaust air is introduced into the intake air, is not considered, and measurements for drawbacks caused by the condensed water in the exhaust air introduction are not sufficiently made. Further, according to JP H7-259656A, JP H7-247917A, and JP H11-210560A, the exhaust introducing path opens to a wall surface of the intake path located around the throttle valve. However, in order to uniformly diffuse the exhaust air into the intake air without causing uneven exhaust airflow between the exhaust introducing path and the cylinders, the exhaust introducing path should open to a position so that the exhaust air is introduced in the intake airflow that has been divided for each cylinder. Moreover, the position should project into the intake airflow to some extent. However, when the exhaust introducing path opens in the position meeting the above-described conditions, the exhaust air tends to be subject to the effects of the intake pulsation. Thus, the uniform diffusion of the exhaust air into the intake air is hindered, and the uniform distribution of the exhaust air to each cylinder becomes difficult.
The same is true for other gases such as blow-by gas or a fuel vapor from a canister. A need exists for a gas introducing structure of an intake path which is not susceptible to the drawback mentioned above.